This invention relates in general to a method and device for determining the forces applied to a support member or cable. In particular, the deflection of the support may be measured to determine the force input to a support member or a payload suspended by the support member. Other embodiments of the invention relate to a method and device for measuring forces applied along a single axis as well as a method and device for determining the orientation of a surface or object.
Motorized gantry cranes as well as non-motorized overhead rail systems may be used to assist a human operator in moving bulky or heavy payloads. In either case, a powered hoist is most commonly used to lift the payload. For large loads supported by a gantry crane, a motor-driven trolley and bridge rail transport the hoist in accordance with the operator""s commands issued through a control box. For smaller loads supported by an overhead rail system the operator may push on the payload directly, causing the free-rolling trolley and bridge rail to follow along passively.
An intuitive interface to the gantry crane would allow better dexterity than is afforded by a control box. An intuitive interface to the overhead rail system would allow the addition of motors without reducing the operator""s dexterity. Thus, an appropriate interface would allow gantry cranes the benefit of the operator""s dexterity and overhead rail systems the benefit of powered motion.
In either case the powered motion of the trolley and bridge rail must reflect the operator""s intent, which is most naturally expressed by pushing directly on the payload. If the payload is suspended by a support means or cable, the degree and direction of its deflection may be used to indicate the force applied to the payload by the operator.
A number of different techniques have been practiced to measure the position or movement of a support means or cable suspending a payload. Typically, mechanical means of detecting the position of the support have been used, such as a moveable wiper arm running against a potentiometer. The resistance varies as the moveable wiper arm moves along the length of the potentiometer.
U.S. Pat. No. 3,982,733 to Orme entitled xe2x80x9cGimbaled Sheave With Cable Angle Sensorsxe2x80x9d describes a system for maintaining the position of a helicopter or waterborne platform over a underwater array suspended from a cable by measuring the angle of the cable. The angle of the cable is measured using a conventional mechanical means, a rotary transformer encoder. The patent describes a design of a gimbaled sheave from which the cable pays out that allows the angle of the sheave to be measured by the rotary transformer encoder to determine the angle of the cable.
U.S. Pat. No. 5,350,075 to Kahlmann entitled xe2x80x9cArrangement For Controlling The Direction And Movement Of A Load Hoist Trolleyxe2x80x9d also describes a mechanical means of measuring the movement of a cable. The movement of the cable is used to determine the force manually applied to a payload suspended from a cable. The force applied by an operator to the payload produces a displacement of the payload and the suspending cable.
Slightly below the hoist, the cable passes through a guide that moves laterally as the cable moves with the movement of the payload. The lateral motion of the guide is measured and gives an indication of the force applied by the operator.
A mechanical device coming in contact with the movable support, however, can constrain and impede the motion of the support and may be easily damaged by a sudden violent or uncontrolled movement of the support. In an industrial application, even a relatively small and controlled movement of a heavy payload can be translated and magnified to a sudden and violent movement of its supporting means. To provide a device capable of withstanding the rigors of such potential shocks and impacts without sustaining damage, an attempt can be made to construct the device of sufficient structure rugged enough to withstand such shocks. Such a durable device of increased ruggedness, however, may be heavy, expensive to build, and still constrain the movement of the support. Thus, to efficiently and reliably detect the motion of the support, a mechanism that does not contact the support is desired.
To this end, a number of optical and electrical means of detecting the position of a support without physically contacting the support have been suggested.
U.S. Pat. No. 5,785,191 to Feddema entitled xe2x80x9cOperator Control Systems And Methods For Swing-Free Gantry-Style Cranesxe2x80x9d describes techniques for eliminating the unwanted swing of a payload suspended from a crane. Gantry-style cranes are usually moved by operating left/right and forward/back push buttons to start and stop the crane. The sudden stops and starts of the crane cause the suspended payload to swing. The patent suggests a non-contact cable angle sensor using a capacitance measurement but does not disclose the suggested cable angle sensor.
U.S. Pat. No. 4,464,087 to Schumann entitled xe2x80x9cInductively Coupled Position Detection Systemxe2x80x9d shows a non-contacting technique for determining the position of a handheld joystick moveable member relative to two or three orthogonal axes. The disclosed system shows an inductively coupled position sensing device having a drive coil affixed to the moveable member of interest and pickup coils which are located to define a pair of intersecting stationary axes, i.e. an X-Y coordinate system with four quadrants. One of the pickup coils is located in each of the quadrants. The pickup coils are arranged and interconnected such that the mathematical sum of the induced voltages will be of a magnitude and polarity indicative of the position of the moveable member. The disclosed device is configured for a joystick application that may be suitable for a force-sensing handle. It is still, however, not suitable for an overhead rail system where the payload is free to rotate and swivel without providing directional information.
Thus, it would be desirable to provide a system capable of improving the detection and measurement of force applied to a suspended payload.
In addition to moving a payload in the lateral direction, there is also a need to control the load in the up-down direction or Z-axis direction. A hoist for lifting heavy objects is typically controlled by a control box having up/down push buttons. To cause the hoist to raise the payload, a button corresponding to raising the payload up is pushed and released.
To lower the payload, a second button corresponding to lowering the payload is pushed and released. Raising and lowering the load through the actuation and release of the control push buttons require the operator to carefully watch the load and time his actuation and release of the push buttons. Thus, it is desirable to provide a more intuitive and direct method of operating the hoist to raise and lower the load.
When the operator""s force in a lateral direction causes the bridge rail of a gantry crane to traverse along its long rails, it is necessary to control the motion of the bridge rail such that it remains perpendicular to the long rails. This may be done by sensing any deviation from perpendicularity and correcting the deviation. The sensing of the skew angle of the bridge rail is subject to similar requirements of ruggedness as those involved in the sensing of lateral movement of the cable, and the sensing of axial forces along the cable. A non-contact sensor is desirable for this purpose.
In accordance with an illustrative embodiment of the invention, problems associated with measuring the deflection of a support member or suspended cable are addressed. The illustrative embodiment of the invention can be used, for example, to measure the magnitude and direction of forces applied to a payload attached to the support member. From the measured deflection of the support member, the forces applied to a payload can be determined. The illustrative embodiment can be used with a gantry crane or overhead rail system to determine an operator""s intent in guiding an object or payload.
In an embodiment of the invention, the deflection angle that the support member deviates away from an initial or neutral position is measured. The initial or neutral position is the position where the forces applied to the support member are at equilibrium or at an initial predetermined level, such as the support resting in a vertical position. In response to an applied force, such as a human operator pushing the payload attached to the support member, the support member will be displaced or deflected from its initial position an amount or magnitude and direction proportional to the applied force. The deflection of the support member from its initial position forms an angle from its initial position. The measured deflection angle indicates the magnitude and direction of the force applied to the payload.
According to an aspect of the invention, mechanical or physical contact with the support is not required. Pickup coils positioned adjacent to the support member detect the angle of the support. In this embodiment, the pickup coils are planar coils oriented relative to the opening through which the support passes such that the plane of the opening and the plane of the pickup coils lie in the same horizontal plane. In a particular embodiment, printed coils in a electronic printed circuit board are used. In other embodiments of the invention, wire wound coils may also be used.
According to another aspect of the invention, the support member has a magnetic field associated with it that interacts with the pickup coils positioned adjacent to the opening. In a preferred embodiment, energizing the support member with an electric alternating current (xe2x80x9cACxe2x80x9d) signal creates the magnetic field. The AC signal carried by the support member creates a fluctuating magnetic field around the support that is parallel to the plane of the pickup coils. When the support member is at an initial position, the magnetic field is parallel to and does not interact with the pickup coils. When the support member is deflected by an applied force, however, it undergoes an angular deviation that causes the magnetic field to undergo a corresponding angular deviation which causes the magnetic field to intersect into the plane of the pickup coils. The interaction of the fluctuating magnetic field with the plane of the pickup coils induces a corresponding voltage in the pickup coils that indicates the angle of deviation of the support member from its initial vertical position.
According to another aspect of the invention, a detector circuit detects the voltage induced in the pickup coils and indicates the angle of deflection of the support member. In a particular embodiment, the detector circuit uses synchronous detection to demodulate the voltage induced in the pickup coils and outputs a direct current (xe2x80x9cDCxe2x80x9d) signal proportional to the angle of the support member.
According to another embodiment of the invention, the problems associated with stray magnetic field affecting measurements are addressed. In a preferred embodiment, a non-ferromagnetic but conductive shield is used to prevent the coils from picking up extraneous signals or noise that may cause spurious readings and affect the cable angle measurements. The shield protects against stray magnetic fields and allows the pickup coils to measure only the magnetic field from the support member.
In yet another embodiment of the invention, the support or cable is energized to carry an AC signal without physical contact with the cable. In a particular embodiment, a toroidal transformer induces an alternating electric current in the support member.
In accordance with another illustrative embodiment of the invention, problems associated with measuring the vertical forces exerted on a support member or cable are addressed. In this embodiment, an operator can exert forces up or down on the cable to direct the hoist to raise and lower the payload. A handle is preferably positioned on the cable coaxial to the cable and surrounding the cable. A one-axis force sensor connected to the handle using a piezoelectric sensor or strain gauge can directly measure the magnitude and direction of applied forces. The measured force applied to the handle in the up and down (vertical) direction can be measured and then used by the hoist to determine whether to raise or lower the payload.
In an embodiment of the invention, a spring is connected to the handle to convert a force applied to the handle into a displacement of the handle. The displacement of the handle can be measured to determine the force applied to the handle. Springs are commonly used to convert force to displacement. Prior art techniques have involved reading this displacement with a potentiometer or optical encoder.
According to a preferred embodiment of the invention, the spring is energized to carry an AC electrical signal. As a force is applied to the handle, the spring compresses, causing the coils of the spring to move closer together. Due to the movement of the spring coils, the inductance of the spring varies according to the force applied to the handle.
Detector circuitry detects the change in inductance of the spring and determines the force and the intent of the operator in raising or lowering the hoist. According to this embodiment, a more durable device to measure forces applied to an handle in the vertical z-axis can be provided.
In accordance with another illustrative embodiment of the invention, problems associated with determining the orientation of an object are addressed. In an embodiment according to this aspect of the invention, a parallelism sensor provides the ability to measure the degree of parallel orientation of an object with respect to another plane or surface.
The preferred embodiment of the parallelism sensor includes a sensor pad having an energized coil flanked by a pair of pickup coils. The energized coil produces a fluctuating magnetic field that is sensed by the pickup coils. When the sensor pad is parallel to a conductive surface, the magnetic flux lines passing through the pickup coils are equal. When the sensor pad is not parallel to the conductive surface, the magnetic flux lines passing through the pickup coils are no longer equal and a voltage is induced in the pickup coil circuitry.
In the illustrative embodiment described herein, the parallel orientation information is supplied as feedback to a motion control system such as one that controls the movement of an overhead rail system.
The preferred embodiments have many uses and advantages. The deflection of a payload or support member can be measured without physical contact with the measured support or payload. This is an advantage because such physical contact may cause damage to the device or impede the motion of the support member. The measured deflection of the support can be utilized to determine the forces applied to a payload attached to the support. In a particular embodiment of the invention, the magnitude and direction of the force indicates the operator""s intent in moving the payload and determines the proper additional force to assist an operator in manipulating the device as desired. According to other embodiments of the invention, the vertical forces applied to a cable can be measured and used to determine whether to raise or lower the payload. The disclosed embodiments provide a durable device that is capable of withstanding the constant handling, shock, and abuse that such a load handle is likely to suffer. According to still other embodiments of the invention, the orientation of an object or surface with respect to a surface can be determined without contact between the two surfaces. This embodiment allows the relative orientation of the two objects to be determined without mechanical assemblies. This is an advantage because such assemblies may constrain the movement of the objects and eventually wear, causing the assemblies to no longer function properly.